This invention relates to dental prosthesis mounted over metal implants anchored to the bone.
The procedure is based upon a titianium part analog to a tooth root which by means of a known and elaborated surgical technique is fixed to the patient's bone. This implant serves as the base upon which a transmucosal intermediate member is attached serving this latter as support for the final prosthesis to be fixed upon.
Given the need to replace natural missing teeth, there has been a large amount of research in the dentistry field leading to the findings of Br.ang.nemark in the 1960's. The present practice starts with a surgical procedure that carefully and with the minimal trauma, places a titanium plug in the patient's bone. This implant is left in place covered by the sutured gum, undisturbed and without any loads being applied to it, for 4 to 6 months prior to the final stage. After that time, a prosthesis is fixed to the anchored implant usually by means of intermediate members.
At the present time there are several types of implant-supported prosthesis systems in use (Core-Vent, Nobelpharma, Calcitek, IMZ, etc.). These systems comprise small plugs, generally cylindrical, sometimes self tapping screws, of 3 to 6 milimeters in diameters and 6 to 22 milimeters in length, that are anchored in the bone. Onto these plugs the prosthesis are fixed by means of different members varying in number, form and function according to the chosen system. A transmucosal spacer member, usually known as an abutment, is generally used to connect the final prosthesis to the implanted anchor. In some cases a gold cylinder is used as the connecting link between the abutment and the final crown, being fixed to the titanium plug by means of a screw and becoming, after casting, an integral part of the prosthesis. In other systems a plastic sheath is used instead of the cylinder; it melts away with the waxing material in the moulding process leaving a cavity for the fixing screw.
The anatomy of each individual patient places limitations on the direction of the implant. The reposition of missing teeth is generally done over patients that have suffered some degree of bone loss. In addition maxillary sinus grows in those places where there are no teeth. The width of the available jaw bones is another determinant factor for the placement of the anchor. The combination of these and other individual features of the patient makes it usually impossible for the direction of the main axis of the implanted plug to be that required for the final prosthesis' alignment.
The first implants made were done only on those patients in which the characteristics of the jaw bone was such that allowed the axis of the implanted anchor be the same as the required by the prosthesis. This fact limited the implants to the anterior part of the lower jaws between the mandibular foramina; and in the upper jaws between maxillary sinuses or where adecuate space was available. These characteristics accounted for the fact that the majority of the early patients were complete endentulous.
Later on, different companies offered abutments that partially compensated for this non alignment of the implanted plug with the prosthesis. They enable the placement of the implanted anchor in places where the direction of its main axis would not be optimal for the final prosthesis. This was accomplished by means of angulated transmucosal spacer members or abutments having the axis of the emergent part in a direction different from that of the implant's axis. Some required intermediate laboratory casting procedures involving many time consuming sesions. Another type of intermediate abutment was to be bent to accommodate the final position. These bendable abutments were criticized because remnant stresses weakened the part. Recently new types of angulated abutment are available in the market. They are preangulated and can be rotated to different positions to achieve the desired direction; afterwards they are fixated.
Of this latter type, Nobelpharma offers an "Angulated abutment, complete, titanium, catalog number SDCA 102." "The angulated abutment has a twelve-sided internal design that locks onto the hexagon on the top of the fixture. This design allows the abutment to be rotated and locked down in twelve different positions on top of the fixture (implanted anchor). This will provide alternatives in order to achieve ideal abutment alignment to compensate for fixtures angled in different directions." This angulated abutment has the shortcoming that is "not intended for single tooth treatment due to the fact that there is no absolute interlock between the gold cylinder and the abutment to prevent rotation". Quoted from "Angulated Abutment, Clinical Procedure and Case Presentation", Nobelpharma 1989.
Similarly, Implant Innovations, Inc. offers a "pre-angled abutment and screw" (Pat. Pending) catalog numbers PA152, PA154, PA156, PA252, PA254, PA256, PA352, PA354, PA356. The following is quoted from Implant Innovations' catalog: "Pure titanium abutment for correction of non-parallel Br.ang.nemark type implants. Available in 15, 25 and 35 degree angle corrections and 2, 4, an 6 mm trans-tissue heights. Can be rotated to 12 different positions on the implant. Non rotational geometry allows for single tooth replacement. Cemented (15 degree) or screw retained (25 and 35 degree). Wax to abutment and cast telescopic crown to fit the abutment." The casting procedure is not so accurate as it is with those systems that use an intermediate gold cylinder telescopically mounted on the abutment. As it is the case with the Nobelpharma's angulated abutment this systems allows only 12 different azimuth positions around the main axis of the implanted anchor.
Core-Vent Corporation offers two kinds of angulated abutments, each of them coming in a series to cover different specific cases. One of these is manually bendable using appropriate tools. This type does not give an precise angulation and is also weakened by remnant stresses. The other kind requires the casting of a custom made abutment, and includes a stainless steel housing for the fixation of the prosthesis by means of a titanium screw. Associated with this type are a larger number of visits to the clinic and a time consuming laboratory procedure which affects the final cost. None of these types is as accurate as those that use pre-angulated parts attached to one another by means of screws.
Besides these mentioned examples, there are others that give partial answer to some of the problems stated earlier.